FIG. 45 is a sectional view illustrating a part of a prior art InP DCBH (Double-Channel Buried-Heterostructure) semiconductor laser disclosed in, for example, Electronics Letters, Vol. 24, No. 24, pp. 1500-1501 (1988). In the figure, reference numeral 201 designates a p type InP substrate. A p type InP lower cladding layer 202 is disposed on the p type InP substrate. An undoped InGaAsP active layer 203 is disposed on the p type InP cladding layer 202. An n type InP first upper cladding layer 204 is disposed on the active layer 203. A stripe shape mesa is formed by selectively etching the InP substrate 201, the p type InP cladding layer 202, the InGaAsP active layer 203, and the n type InP first upper cladding layer 204. A p type InP layer 205 is disposed on opposite sides of the mesa. An n type InP current blocking layer 206 is disposed on the p type InP layer 205. A p type InP current blocking layer 207 is disposed on the n type InP current blocking layer 206 and on portions of the p type InP layer 205. An n type InP second upper cladding layer 208 is disposed on the n type InP first upper cladding layer 204 and on the p type InP current blocking layer 207. An n type InGaAsP contact layer 209 is disposed on the n type InP second upper cladding layer 208. An n side electrode 211 is disposed on the n type InGaAsP contact layer 209, and a p side electrode 210 is disposed on the rear surface of the p type InP substrate 201. Although the semiconductor laser disclosed in this literature includes a light guide layer having a diffraction grating on the active layer, the light guide layer is omitted in FIG. 45.
FIGS. 46(a)-46(d) are perspective views illustrating process steps of fabricating the optical waveguide of the semiconductor laser shown in FIG. 45. In the figures, the same reference numerals as in FIG. 45 designate the same or corresponding parts.
Initially, as illustrated in FIG. 46(a), there are successively grown on the p type InP substrate 201 with a {100} surface orientation, the p type InP layer 202 about 1 .mu.m thick, the InGaAsP active layer 203 about 0.1 .mu.m thick, and the n type InP first upper cladding layer 204 about 1 .mu.m thick. Preferably, these layers are grown by metal organic chemical vapor deposition (MOCVD).
Then, a negative photoresist 215 is deposited on the n type InP first upper cladding layer 204 and patterned by photolithography, forming a stripe pattern extending along a &lt;011&gt; direction and having a width of about 6 .mu.m (FIG. 46(b)). Since an oxide film is deposited on the surface of the n type InP first upper cladding layer 204 after the crystal growth, the deposition of the photoresist 215 should be carried out after removing the oxide film from the surface using hydrofluoric acid.
Using the stripe pattern 215 as a mask, the wafer is etched until the etching front reaches into the substrate 201, forming a ridge-shaped optical waveguide with 1-2 .mu.m wide active layer 202 (FIG. 46(c)). Preferably, a mixture of bromine (Br.sub.2) and methanol (CH.sub.3 OH) is employed as an etchant.
After removing the photoresist pattern 215, the p type InP layer 205, the n type InP current blocking layer 206, and the p type InP current blocking layer 207 are successively epitaxially grown on the substrate 201 contacting opposite sides of the optical waveguide (FIG. 46(d)). Preferably, these layers are grown by liquid phase epitaxy (LPE).
Thereafter, the n type InP second upper cladding layer 208 and the n type InGaAsP contact layer 209 are successively epitaxially grown over the entire surface of the wafer. To complete the laser structure of FIG. 45, the p side electrode 210 and the n side electrode 211 are formed on the rear surface of the substrate 201 and on the contact layer 209, respectively.
FIGS. 47(a)-47(d) are perspective views illustrating improved process steps of fabricating the laser structure of FIG. 45 in which unwanted growth of the p type InP current blocking layer on the ridge structure, that is likely to occur in the process steps of FIGS. 46(a)-46(d), is prevented.
In this method, after successively growing the p type InP buffer layer 202, the InGaAsP active layer 203, and the n type InP cladding layer 204 on the p type InP substrate 201 (FIG. 47(a)), an SiO.sub.2 film 216 is deposited on the wafer by sputtering or the like and patterned in a stripe shape extending along the &lt;01122 direction (FIG. 47(b)). Then, using the SiO.sub.2 pattern 216 as a mask, the stripe shape ridge is formed along the &lt;011&gt; direction using the same etchant as described above (FIG. 47(c)).
Thereafter, using the SiO.sub.2 pattern 216 as a mask, the p type InP layer 205, the n type InP current blocking layer 206, and the p type InP current blocking layer 207 are successively grown on regions of the wafer that are not masked with the SiO.sub.2 pattern 216 (FIG. 47(d)).
After removing the SiO.sub.2 mask 216 with hydrogen fluoride, the n type InP second upper cladding layer 208 and the n type InGaAsP contact layer 209 are successively epitaxially grown over the entire surface of the wafer. To complete the laser structure of FIG. 45, the p side electrode 210 and the n side electrode 211 are formed on the rear surface of the substrate 201 and on the contact layer 209, respectively.
A description is given of the operation.
When a forward bias voltage is applied across the electrodes 210 and 211, a current flows between the electrodes, and electrons and holes are injected into the InGaAsP active layer 203 from the n type InP upper cladding layer 204 and the p type InP lower cladding layer 202, respectively. The electrons and holes injected into the active layer 203 recombine to produce light, resulting in laser oscillation. Since the p type InP layer 205, the n type InP current blocking layer 206, and the p type InP current blocking layer 207 are present on opposite sides of the ridge structure, the InGaAsP active layer 203 is buried with InP crystal having a refractive index smaller than that of InGaAsP, so that the light generated in the active layer 203 is effectively confined in the active layer 203. In addition, since a reverse bias 3unction is produced by the n type InP current blocking layer 206 and the p type InP current blocking layer 207 at opposite sides of the ridge structure, the current flow path is narrowed and the charge carriers are injected into the active layer 203 with high efficiency.
In the laser structure shown in FIG. 45, however, a reactive current path (first reactive current path) is formed only by the forward biased junction across the p type InP substrate 201, the p type InP layer 205, and the n type InP cladding layer 204. In this case, all of the current injected into the laser does not flow into the InGaAsP active layer 203, but a part of the current flows through the first reactive current path, and does not contribute the laser oscillation. The reactive current adversely affects the oscillation threshold, the maximum output, and the temperature characteristics of the laser device. The amount of the reactive current flow depends on the resistance of the first reactive current path. Therefore, in order to attain a semiconductor laser with less reactive current and improved characteristics, it is necessary to increase the resistance of the first reactive current path by narrowing the width of the first reactive current path. The narrow width of the reactive current path is achieved by reducing the thickness of the p type InP layer 205 on opposite sides of the active layer 203, i.e., by reducing the space between the active layer 203 and the n type InP current blocking layer 206.
If the n type InP current blocking layer 206 is in contact with the n type InP cladding layer 204, a second reactive current path is formed by the forward biased junction across the p type InP substrate 201, the n type InP current blocking layer 206, and the n type InP cladding layer 204 as shown in FIG. 49, in addition to the first reactive current path. The unwanted contact between the n type InP current blocking layer 206 and the n type InP cladding layer 204 occurs when the thickness of the p type InP layer 205 on the upper side surface of the ridge is thin. If the thickness of the p type InP layer 205 on opposite sides of the active layer 203 is reduced to reduce the resistance of the first reactive current path, on the other hand, the thickness of the InP layer 205 on the upper side portions of the ridge is also reduced, whereby the n type InP current blocking layer 206 is unfavorably in contact with the n type InP cladding layer 204, resulting in the second reactive current path.
Accordingly, in order to attain a semiconductor laser with less reactive current and improved characteristics, it is necessary to precisely control the thickness of the p type InP layer 205 when growing the layer.
However, the LPE method used for the fabrication of the conventional semiconductor laser does not offer good controllability of the layer thicknesses, so that a semiconductor laser with less reactive current and improved characteristics is not achieved by the conventional method.
Although MOCVD offers good controllability of the layer thicknesses in contrast with LPE method, it has been unknown whether the n type InP current blocking layer 206 can be grown without contacting the n type InP cladding layer 204 by MOCVD. Therefore, MOCVD has never been employed for the growth of the layer 206.
In the production method illustrated in FIGS. 47(a)-47(d), when the stripe shape ridge is formed by etching using the SiO.sub.2 film 216 as a mask, since the adhesion of the SiO.sub.2 film 216 to the upper cladding layer 204 is high, the etching does not proceed in the transverse direction under the SiO.sub.2 mask 216, and the upper end part of the ridge is shaped in a reverse mesa with (111)A surfaces at the side walls thereof as shown in FIG. 50. During the LPE process after the etching process, the growth of the p type InP layer 205 does not proceed on the side surfaces of the upper end of the ridge where the (111)A surfaces are present, resulting in the unwanted contact between the n type InP current blocking layer 206 and the n type InP clad layer 204. The unwanted contact causes the second reactive current path. Therefore, a semiconductor laser with improved characteristics is not achieved in this method. In addition, this production method provides uneveness of the grown crystal layer, which adversely affects the subsequent process of forming electrodes or the like, reducing the production yield.
The above-described problems due to the reverse mesa shaped portion with the (111)A surfaces occur also in the production method illustrated in FIGS. 46(a)-46(d) where the ridge structure is formed using the photoresist 215 as a mask.
The cross section of the optical waveguide formed by the etching process shown in FIG. 46(c), which etching process is carried out using the stripe pattern extending along the &lt;011&gt; direction as a mask and the Br.sub.2 :CH.sub.3 OH system solution as an etchant as an etchant, depends on the adhesion of the photoresist to the surface of the n type InP upper clad layer 204. In order to attain the ideal shape of the current blocking layer 205 as shown in FIG. 46(d), the cross section of the optical waveguide formed by the etching process must be in a gently-sloping mesa shape (ordinary mesa shape) as shown in FIG. 46(c). However, this gently-sloping mesa shape is attained when the adhesion of the photoresist to the surface of the upper clad layer 204 is low and the etching proceeds in the transverse direction under the photoresist mask. On the other hand, when the adhesion of the photoresist is high, since the side-etching rate under the photoresist mask is small, the cross section of the optical waveguide is in the reverse-mesa shape as shown in FIG. 50. If the current blocking layers 205, 206, and 207 are epitaxially grown by LPE on the wafer etched as shown in FIG. 50, the growth rate on the side walls of the reverse-mesa portion is low, resulting in the structure of FIG. 51 in which the n type InP current blocking layer 206 is in contact with the n type InP upper cladding layer 204. In this structure, the second reactive current path is formed and the leakage current increases during the laser operation.
FIG. 52 is a sectional view illustrating a prior art semiconductor laser disclosed in, for example, Japanese Published Patent Application No. 63-169088. In the figure, reference numeral 221 designates a p type InP substrate. A p type InP buffer layer 222 is disposed on the substrate 221. An InGaAsP active layer 223 is disposed on the buffer layer 222. An n type InP first upper cladding layer 224 is disposed on the active layer 223. The first upper cladding layer 224, the active layer 223, and the buffer layer 222 are selectively etched to form a stripe-shaped mesa structure 225. The mesa structure is embedded with a first p type InP layer 226, an n type InP layer 227, and a second p type InP layer 228. An n type InP second upper cladding layer 229 is disposed on the mesa structure and on the second p type InP layer 228.
In production, the buffer layer 222, the active layer 223, and the first upper cladding layer 224 are successively grown on the p type InP substrate 221, and portions of these layers 222, 223, and 224 are formed in a mesa shape by etching. Then, LPE is carried out to embed both sides of the mesa with the first p type InP layer 226, the n type InP layer 227, and the second p type InP layer 228. Finally, the second upper cladding layer 229 is grown over the surface of the wafer to complete the structure of FIG. 52.
A description is given of the operation.
In the laser structure of FIG. 52, when a forward bias voltage is applied across the p type InP substrate 221 and the n type InP upper cladding layer 229, charge carriers, i.e., holes and electrons, are injected into the InGaAsP active layer 223 and recombine to generate light, resulting in laser oscillation. Since the first p type InP layer 226, the n type InP layer 227, and the second p type InP layer 228 are present on opposite sides of the mesa structure, the InGaAsP active layer 223 is embedded with InP having a refractive index smaller than that of InGaAsP, whereby the light generated in the active layer is effectively confined in the active layer 223. In addition, since a reverse bias junction is formed by the n type InP layer 227 and the p type InP layer 228 at opposite sides of the active region, the current path is narrowed and the charge carriers are injected in the active layer 203 with high efficiency.
In the semiconductor laser of FIG. 52, however, since the n type InP first upper cladding layer 224 is in contact with the n type InP layer 227, a second reactive current path 230 is formed across the p type InP substrate 221, the p type InP layer 226, the n type InP layer 227, and the n type InP first upper cladding layer 224. The amount of the current that does not flow into the active layer 223 but flows through the second reactive current path 230 is negligible during the low power operation of the laser because the built-in potential at the heterojunction in the active layer is smaller than the built-in potential at the homojunction in the reactive current path 230. However, it is a serious problem during the high power operation of the laser.
Accordingly, in order to achieve a high power operation of the laser, it is very important to separate the n type InP first upper cladding layer 224 from the n type InP layer 227. A method for achieving this separation is disclosed in Japanese Published Patent Application No. 63-202985. The separation method is illustrated in FIG. 54. In FIG. 54, end portions of the n type InP layers 227 are converted to p type utilizing the mutual diffusion of impurities between the n type InP layer 227 and the p type InP layer 226 or 228. Thus formed p type regions 227' electrically separate the n type InP first upper cladding layer 224 from the n type InP layers 227.
In this method, however, there are limitations in designing the respective layers 226, 227 and 228. In the structure of FIG. 54, the current narrowing structure for effectively injecting current into the active layer 223 is achieved by the p-n-p-n thyrister comprising the first p type InP layer 226, the n type InP layer 227, the second p type InP layer 228, and the n type InP second upper cladding layer 229. In order to realize high power operation in this laser structure, the breakdown voltage of the p-n-p-n thyrister must be high. In order to increase the breakdown voltage of the p-n-p-n thyrister, the injection of holes from the first p type InP layer 226 into the second p type InP layer 228 has to be suppressed by increasing the carrier concentration of the n type InP layer 227 for recombination of holes with electrons in the n type InP layer 227. In the above-described method utilizing the mutual diffusion of impurities, however, if the carrier concentration of the n type InP layer 227 is increased, the carrier concentration of the p type InP layer 226 or 228 has to be increased to convert the end portion of the n type InP layer 227 to p type. When Zn is employed as the p type impurity, it is difficult to obtain a p type carrier concentration higher than 3.times.10.sup.18 cm.sup.-3. Therefore, if the carrier concentration of the n type InP layer 227 is increased, the conversion of the end portion of the n type InP layer 227 to p type is impossible. In addition, if the carrier concentration of the p type InP layer 226 or 228 is increased to convert the end portion of the n type InP layer 227 to p type, the impurity dopant diffuses not only into the n type InP layer 227 but also into the active layer 223, whereby the absorption loss of the light generated in the active layer 223 increases due to free carrier absorption, increasing the oscillation threshold of the laser device.
Accordingly, in the laser structure of FIG. 54, since the carrier concentration of the n type InP layer 227 cannot be sufficiently increased, the breakdown voltage of the p-n-p-n thyrister structure cannot be increased.
In the structure shown in FIG. 55 in which the n type InP layer 227 is separated from the n type InP first upper cladding layer 224, a first reactive current path 231 is formed across the p type InP layer 226 and the n type InP first upper cladding layer 224. Since the resistance of the p type InP is larger than that of the n type InP, the amount of the current flowing through the first reactive current path 231 is less than the amount of the current flowing through the second reactive current path 230 in the laser structure shown in FIG. 53 in which the n .type InP layer 227 is in contact with the n type InP first upper cladding layer 224. However, the current flowing through the first reactive current path 231 adversely affects the high output characteristics of the laser. In order to decrease the current flowing through the reactive current path 231, the resistance of the reactive current path 231 should be increased. As means for increasing the resistance, reduction in the carrier concentration of the p type InP layer 226 and reduction in the width 232 of the reactive current path 231 by narrowing the space between the active layer 223 and the n type InP layer 227 are thought of. However, the former is not effective for reducing the reactive current because the built-in potential of the p-n junction formed by the n type InP first upper cladding layer 224 and the p type InP layer 226 is unfavorably reduced. Accordingly, the reduction in the width 232 of the leakage current path is important for attaining high output characteristics of the laser. FIG. 56 illustrates the leakage current path width dependence of maximum output power (P.sub.max). As shown in the figure, the high output characteristics of the laser significantly depend on the leakage current path width 232.
FIG. 57 is a perspective view illustrating an InGaAsP buried heterojunction type semiconductor laser using an n type substrate disclosed in, for example, Journal of Lightwave Technology, Vol. 7, No. 10, October 1989, p. 1515. FIG. 58 is a sectional view of the semiconductor laser of FIG. 57. In these figures, reference numeral 241 designates an n type InP substrate. An n type InP lower cladding layer 242 having a stripe ridge is disposed on the substrate 241. An undoped InGaAsP active layer 243 is disposed on the stripe ridge of the lower cladding layer 242. A p type InP first upper cladding layer 244 is disposed on the active layer 243. A p type InP current blocking layer 245 is disposed on the lower cladding layer 242 at opposite sides of the ridge. An n type InP current blocking layer 246 is disposed on the p type current blocking layer 245. A p type InP second upper cladding layer 247 is disposed on the p type InP first upper cladding layer 244 and on the n type InP current blocking layer 246. A p type InGaAsP contact layer 248 is disposed on the second upper cladding layer 247. An n side electrode 249 is disposed on the rear surface of the substrate 241, and a p side electrode 250 is disposed on the contact layer 248. Although a lightguide layer including a diffraction grating is disposed on the active layer in the above-described literature, the lightguide layer is omitted in the FIGS. 57 and 58.
A method for fabricating the laser structure is illustrated in FIGS. 59(a)-59(c).
Initially, there are successively grown on the n type InP substrate 241 the n type InP cladding layer 242, the undoped InGaAsP active layer 243, and the p type InP cladding layer 244 by MOCVD, and an SiO.sub.2 film 251 is deposited over the p type InP cladding layer 244 by sputtering and patterned in a stripe shape by conventional photolithography (FIG. 59(a)).
Using the SiO.sub.2 film 251 as a mask and a Br.sub.2 :CH.sub.3 OH solution as an etchant, the wafer is selectively etched as shown in FIG. 59(b). Then, the p type InP current blocking layer 345 and the n type InP current blocking layer 249 are successively grown on the n type InP cladding layer 242 at opposite sides of the mesa structure by MOCVD (FIG. 59(c) ).
After removing the SiO.sub.2 film 252 with HF, the p type InP cladding layer 247 and the p type InGaAsP contact layer 248 are successively grown on the wafer by MOCVD. The laser structure shown in FIG. 57 is completed by forming the n side electrode 249 and the p side electrode 250 on the rear surface of the substrate 241 and on the contact layer 248, respectively.
A description is given of the operation. When a forward bias voltage is applied across the n type InP substrate 241 and the p type InGaAsP contact layer 248 from the electrodes 249 and 250, respectively, charge carriers, i.e., holes and electrons, are injected into the InGaAsP active layer 243 and recombine to generate light, resulting in laser oscillation. Since both sides of the InGaAsP active layer 243 are embedded with the InP current blocking layers 245 and 246 having the refractive index smaller than that of InGaAsP, light generated in the active layer 243 is effectively confined in the active layer. In addition, since a reverse bias junction is formed by the n type InP layer 227 and the p type InP layer 228 at opposite sides of the active region, the current path is narrowed and the charge carriers are injected in the active layer 203 with high efficiency.
In the conventional method for fabricating the DCBH type semiconductor laser, since LPE that offers poor controllability of the layer thicknesses is employed, semiconductor lasers with less reactive current and improved characteristics are not attained with high reliability.
In the conventional method for fabricating the InP semiconductor lasers, the oxide film formed on the surface of the crystal layer is removed by hydrofluoric acid before the deposition of photoresist. However, the conditions of the oxide films sometimes differ with wafers due to the differences in the time elapsed and the atmosphere after the conclusion of the crystal growth. In addition, the condition of the oxide film formed on a wafer is not uniform in some cases. Therefore, it is difficult to make the crystalline surface uniform with hydrofluoric acid, resulting in uneven adhesion of the photoresist to the crystalline surface that causes an uneven cross section in the waveguide after the etching process. In this case, it is difficult to conduct the subsequent growth of crystal layers to bury the mesa structure with high reproducibility, resulting in poor production yield.
In the conventional buried heterojunction semiconductor laser employing the p type substrate, in order to achieve high output characteristics, the carrier concentration of the n type InP layer 227 should be increased, and the n type InP layer 227 should be close to the active layer 223 and separated from the n type InP first upper cladding layer 224. However, in the conventional method utilizing the mutual diffusion of impurities, the carrier concentration of the n type InP layer 227 cannot be sufficiently increased. In addition, in the crystal growth utilizing LPE, the space between the n type InP layer 227 and the active layer 223 cannot be precisely controlled. As the result, semiconductor lasers with improved characteristics are not achieved with high uniformity and reproducibility.
In the conventional buried heterojunction semiconductor laser employing the n type substrate shown in FIG. 57, since the p type InP cladding layer 244 contacts the p type InP current blocking layer 245, a reactive current path 225 is formed across the p type InP cladding layer 244, the p type InP current blocking layer 245, and the n type InP cladding layer 242 as shown in FIG. 58. The current not flowing into the active layer but flowing through the path 225 may be negligible during the low output operation of the laser because the built-in potential at the heterojunction in the active layer is smaller than the built-in potential of the homojunction in the reactive current path 255. However, the reactive current adversely affects the high output operation of the laser. In the structure of FIG. 58, the p type InP current blocking layer 245 is higher than the active layer 243, and no increase in the resistance of the reactive current path 255 is achieved, resulting in poor characteristics at the high power operation.